Polyimides, particularly aromatic polyimides, have found extensive use in microelectronic devices due to their high thermal stability, low dielectric constant and high electrical resistivity. Aromatic polyimides typically are formed by dissolving an aromatic diamine in a compatible solvent and then adding a dianhydride which reacts with the diamine to make a solution of polyamic acid. The resulting solution of polyamic acid can be coated onto a substrate, the solvent evaporated, and the polyamic acid imidized. Conventional solvents for forming the polyamic acid and subsequent imidization include polar aprotic solvents such as N-methylpyrrolidinone (NMP).
A multilayer interconnect structure, such as a microelectronic multichip module, consists of alternate layers of a conducting material and a material having a low dielectric constant (an insulating material). In particular, polyimides containing fluorine provide a material with a low dielectric constant and low moisture absorption while maintaining desirable features of polyimides such as excellent thermal stability, high glass transition temperature (Tg), and good solvent resistance.
Typically, thick polyimide layers are formed from a number of thin coats of polyamic acid. Each thin coat is imidized before the next coat is applied. However, fluorinated polyimides are degraded by many conventional polar aprotic solvents, such as NMP. It has been observed that these conventional polar aprotic solvents interact with the polyimide and result in cracking of thick layers of the polyimide.
To eliminate the cracking problem a number of approaches can be taken. One approach would be to select polyimides such that the polyimide has a linear coefficient of thermal expansion that matches that of the substrate. This would minimize the stress present in the polyimide film after curing and can make the polymer less susceptible to cracking or crazing. However, few polyimide structures have the very low linear coefficient of expansion required to match typical substrates. Further, these polyimides may not exhibit the low dielectric constant and low moisture absorption that is desirable in electronic applications.
Another approach is to select an alternative solvent system that is less aggressive toward the polyimide. The alternative solvent must not be so volatile that it will evaporate before the polymer solution can be applied to the substrate. It has been found that solvents having a boiling point greater than abort 150.degree. C. will spread to coat the substrate without undue evaporation. Further, the polymer must be sufficiently soluble in the solvent or solvent system for multilayer processing. In other words, the solvent must be able to keep the polymer solvated during the coating of the substrate. The polymer solution should have relatively stable solution viscosity. Finally, the solvent must not attack the underlying polyimide during the multilayer processing steps.